Conventionally, fractional horsepower electric motors have been made with a stator shell and two end shields, with rabbets machined in them to insure proper cover-fit, i.e. concentricity of the central bore of the stator and the bearings carried by the end shields. Such a construction is expensive, because of the machining step, and, in large scale production, requires the provision of a relatively wide annular air gap between the stator and the rotor, because the location of the rotor with respect to the stator is determined by the relation of the shell and end shield rabbets rather than by the relation of the stator and the rotor. On the other hand, such a construction has the advantage of providing a motor which can be disassembled and reassembled readily, to permit repair and replacement of elements of the motor.
Recently, motors have been produced commercially by cementing end shields to the stator core while maintaining the desired air gap spacing with shims, which, after the cementing has been accomplished, are removed. An illustrative example of such a method is described in Thompson and Wightman U.S. Pat. No. 3,165,816, and a resulting motor is described in Thompson and Wightman U.S. Pat. No. 3,176,172. This method permits the commercial production of cheap motors with a small, uniform air gap between the stator and rotor. Such motors suffer from the disadvantages, however, that if the parts are not properly prepared, or if the adhesive application and curing steps are not properly carried out, the motors can debond upon impact in transit or vibration in use; if properly assembled, they can not economically be disassembled once the cement is cured. Accordingly, defects which could readily be cured in a conventionally constructed motor require that the entire motor be scrapped in the case of a cemented motor.
A motor having the excellent concentricity of stator and rotor of a cemented motor and the repairability of a conventional electric motor is described in Arnold U.S. Pat. No. 3,437,853. In this motor, a bridge of cured adhesive material is adhered to either the stator or end shield and is free from adhesion to the other part, the adhesive material constituting the sole radial locating means between the stator and the end shield. The bridge of adhesive is formed while the rotor and stator are held concentrically by shims in the air gap between them. The end shields are held axially in place by conventional through-bolts. Unfortunately, this construction has proven even more subject to misalignments caused by impact than the fully cemented motor. Its assembly also requires both a cementing operation and a bolting operation, and is thus somewhat more complex than that of a fully cemented motor.